The present invention relates to aqueous polymer dispersions which have been stabilized electrostatically by means of anionic groups and which may be obtained, for example, by emulsion polymerization, to processes for preparing them, and to their use.
It is known that, thermodynamically, latices constitute unstable systems which must be stabilized as effectively as possible by means of appropriate measures. One possibility for stabilizing with respect to formation of gel specks or coagulation induced by shearing load, addition of electrolyte or a particularly high solids content may be achieved by copolymerizing monomers containing acidic groups. In general, inorganic bases, such as alkali metal hydroxides or ammonia, are used for neutralizing these acidic groups. From DE-A-19 56 509, dispersions are also known which have been neutralized with specific alkanolamines (namely monohydroxyalkyl-substituted ammonia derivatives) as counterions.
Surprisingly it has now been found that neutralization of acidic groups present on the latex particle surface with aminated sugar alcohols leads to polymer dispersions which are significantly more stable than the prior art.
The present application accordingly provides aqueous polymer dispersions comprising latex particles which contain anionic groups, which are characterized in that at least some of the anionic groups have aminated sugar alcohols in protonated form as counterion.
Preferred aminated sugar alcohols used are N-alkylglucamines or D-glucamine, with particular preference N-alkylglucamines in which the alkyl group has from 1 to 4 carbon atoms, and especially N-methylglucamine. Furthermore, it is also possible to use diglucamines.
The neutralization of an acidic polymer dispersion, which is intended to bring about full charge dissociation and hence stabilization, is normally a critical process step, owing to the high local ion concentration that occurs, and is a step during which a greater or lesser number of gel specks are formed or even, in the worst case, complete coagulation occurs.
If, in contrast, aminated sugar alcohols, preferably N-alkylglucamine, especially N-methylglucamine (NMG), or at least a portion of this aminated sugar alcohol, are or is used as base for the partial or complete neutralization of the polymer dispersions, then a distinct stability advantage is evident in comparison with the products obtained with the neutralizing agents that have been customary to date: the determination of the coagulum content of a latex neutralized with different bases, by filtration, in fact always gives the lowest sieve residue figures for products treated with aminated sugar alcohols, irrespective of the monomer system used for the polymerization.
A further advantage is the renewable raw material basis of these compounds, which are obtainable directly from the corresponding carbohydrates by reductive amination.
The aminated sugar alcohols in question, obtainable starting from glucose, which are described, for example, in WO 93/03004, and which are normally used industrially as raw materials for surfactants (DE-A-35 38 451), are readily soluble in water and are also commercially available in this form.
The advantages of the polymer dispersions of the invention lie in the significantly increased stability as compared with the conventional products, which has positive consequences in connection with the processing of the dispersions to end products, such as inks, plasters, paints or adhesives.
Thus the formation of coagulum or gel specks which is frequently observed on the subsequent addition of pigments, electrolytes, thickeners, etc., and shearing loads occurring in parallel, is greatly restricted or does not occur at all. This advantage, moreover, has no adverse effects whatsoever on the performance properties of the products.
The dispersions of the invention are preferably prepared simply by stirring an aqueous solution (from 5 to 70% in strength) of the aminated sugar alcohol in question into the latexxe2x80x94which is acidic or has already been partly neutralized with other bases. The temperature here is generally between 0xc2x0 C. and 100xc2x0 C., preferably between 20xc2x0 C. and 80xc2x0 C. Also possible, of course, is the stirred introduction of the latexxe2x80x94which is acidic or has already been partially neutralized with other basesxe2x80x94into the aqueous solution of the aminated sugar alcohol in question.
The monomer system on which the polymer dispersion is based, and monomer ratio, is of minor importance, as is the nature of the preceding process for preparing the latex (reemulsification of dispersion powder, emulsion polymerization, suspension polymerization, etc.).
The aqueous polymer dispersions of the invention find application preferably in inks, plasters, paints, and adhesives.